I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for performing signal combining during soft handoff in a wireless communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Patent No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Patent No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
A useful method of power control of a mobile in a communication system is to monitor the power of the received signal from the mobile station at a base station. The base station in response to the monitored power level transmits power control bits to the mobile station at regular intervals. A method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Patent No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a remote station to a central base station is to allow the remote station to send the data using spread spectrum techniques of CDMA. One method that is proposed is to allow the remote station to transmit its information using a small set of orthogonal channels, this method is described in detail in copending U.S. patent application Ser. No. 08/886,604, entitled xe2x80x9cHIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEMxe2x80x9d, now U.S. Pat. No. 6,396,804 issued May 28, 2002, assigned to the assignee of the present invention and incorporated by reference herein.
The present invention is a novel and improved method and apparatus describing the combining of signals in a high rate wireless communication system. In the exemplary embodiment, each base station in communication with a remote station transmits forward link data including traffic data, pilot symbols and overhead data. In the exemplary embodiment, the overhead data includes a reverse link busy bit, reverse link power control (RPC) commands and a forward link activity (FAC) bit. The reverse link busy bit indicates when the base station has reached its reverse link capacity limit. The RPC bit indicates to each mobile station in communication with the base station whether their transmission energy should be increased or decreased. The FAC bit is a message that indicates when a base station will have no forward link data to transmit a predetermined number of slots in the future.
In the exemplary embodiment of the present invention, the forward link traffic is only transmitted from one base station to a given remote station. Thus, there is no soft handoff of the forward link traffic data. The multipath components of the forward link traffic data are combined using a traditional RAKE receiver to provide an improved estimate of the forward link traffic data.
In the exemplary embodiment of the present invention, the reverse link busy bits are independently generated by each base station and indicative of whether the transmitting base station has reached a reverse link capacity limit. In a first exemplary embodiment, the remote station combines the multipath components of the reverse link busy bits from each of the transmitting base stations in its Active Set and in response transmits a reverse link signal only when all of the reverse link busy bits indicate that the base stations in the remote stations Active Set have reverse link capacity. In a first alternative embodiment, the remote station weights the reverse link busy signals in accordance with the signal strength of the base station transmitting the busy signal and determines whether to transmit based on the weighted sum of the busy signals. In a second alternative embodiment, the remote station weights the reverse link busy signals in accordance with the signal strength of the base station transmitting the busy signal and determines a maximum reverse link data rate based on the weighted sum of the busy signals.
In the exemplary embodiment, the FAC signals are independently generated. The FAC signals from common base stations, multipath components, are soft combined and decoded. Each of the FAC signals is provided to a corresponding SNR calculator for each base station. The calculated SNR for each base station is used to determine which base station should transmit forward link data to the remote station and at what data rate.